Expression of the CD46 antigen, and absence of class I MHC antigen, on the human oocyte and preimplantation blastocyst.

Expression of CD46 and class I major histocompatibility complex (MHC) antigens by human oocytes and 6-8-day unhatched expanded preimplantation blastocysts has been studied by immunocytochemistry. The CD46 antigen, a cell surface complement regulatory protein, was expressed by unfertilized oocytes as well as strongly by both the inner cell mass and trophectoderm of preimplantation blastocysts. In contrast, class I MHC antigens were not usually expressed by either oocytes or blastocysts. These data support the concept that gametes and embryonic cells involved in fertilization and early implantation events, respectively, may be protected from immunological recognition or attack both by the lack of class I MHC antigens and by expression of the CD46 complement regulatory protein.